Fix handling of config values

https://github.com/kivra/giallo_session/pull/7 fixed a problem where the 'ttl' setting wasn't read properly from the config.  However, it broke handling of the 'ttl' setting if the default value is used (if that setting is not specified in the config), and it turns out the same issue applies to all of the config settings, not just 'ttl'.

The real issue is that application:gen_env/2 returns {ok, Value} but giallo_session_config:get_env/2 is assuming it simply returns the Value.

This PR fixes giallo_session_config:get_env and reverts the change in PR #7